Soybeans are rich in various bioactive plant compounds, including (, , , ):

Isoflavones. A family of antioxidant polyphenols, isoflavones have a variety of health effects.

Phytic acid. Found in all plant seeds, phytic acid (phytate) impairs the absorption of minerals like zinc and iron. Levels of this acid can be reduced by boiling, sprouting, or fermenting the beans.

Saponins. One of the main classes of plant compounds in soybeans, saponins have been found to reduce cholesterol in animals.

Isoflavones

Soybeans contain higher amounts of isoflavones than other common foods ().

Isoflavones are unique phytonutrients that resemble the female sex hormone estrogen. In fact, they belong to a family of substances called phytoestrogens (plant estrogens).

The main types of isoflavones in soy are genistein (50%), daidzein (40%), and glycitein (10%) ().

Some people possess a special type of gut bacteria that can convert daidzein to equol, a substance considered responsible for many of the beneficial health effects of soybeans.

People whose bodies can produce equol are expected to benefit much more from soy consumption than those whose bodies cannot ().

The percentage of equol producers is higher in Asian populations and among vegetarians than in the general Western population (, ).

SUMMARY Soybeans are a rich source of various bioactive plant compounds, including isoflavones, saponins, and phytic acid. Isoflavones in particular mimic estrogen and are responsible for many of soybeans’ health effects.

Soy products do not affect all women in this way. Soy only seems to be effective in so-called equol producers — those who possess a type of gut bacteria able to convert isoflavones into equol.

Equol may be responsible for many of soy’s health benefits.

Daily intake of 135 mg of isoflavones for 1 week — equivalent to 2.4 ounces (68 grams) of soybeans per day — reduced menopausal symptoms only in equol producers ().

While hormonal therapies have traditionally been used as a treatment for menopausal symptoms, isoflavone supplements are widely used today ().

Bone health

Osteoporosis is characterized by reduced bone density and an increased risk of fractures, especially in older women.

Consumption of soy products may reduce the risk of osteoporosis in women who have undergone menopause (, ).

These beneficial effects seem to be caused by isoflavones (, , , ).

SUMMARY Soybeans contain plant compounds that may help prevent breast and prostate cancer. What’s more, these legumes may relieve menopause symptoms and cut the risk of osteoporosis in postmenopausal women.

Even though soybeans have a number of health benefits, some individuals need to limit their consumption of soy products — or avoid them altogether.

Suppression of thyroid function

High intake of soy products may suppress thyroid function in some people and contribute to hypothyroidism — a condition characterized by low production of thyroid hormones ().

The thyroid is a large gland that regulates growth and controls the rate at which your body expends energy.

Animal and human studies indicate that the isoflavones found in soybeans may suppress the formation of thyroid hormones (, ).

One study in 37 Japanese adults showed that eating 1 ounce (30 grams) of soybeans every day for 3 months caused symptoms related to suppressed thyroid function.

The symptoms included discomfort, sleepiness, constipation, and thyroid enlargement — all of which disappeared after the study ended ().

Another study in adults with mild hypothyroidism found that taking 16 mg of isoflavones every day for 2 months suppressed thyroid function in 10% of the participants ().

The amount of isoflavones consumed was rather small — equivalent to eating 0.3 ounces (8 grams) of soybeans per day ().

However, most studies in healthy adults have not found any significant links between soy consumption and changes in thyroid function (, , ).

An analysis of 14 studies noted no significant adverse effects of soybean consumption on thyroid function in healthy adults, whereas infants born with thyroid hormone deficiency were considered at risk ().

In short, regular consumption of soy products or isoflavone supplements may lead to hypothyroidism in sensitive individuals, especially those who have an underactive thyroid gland.

Flatulence and diarrhea

Like most other beans, soybeans contain insoluble fibers, which may cause flatulence and diarrhea in sensitive individuals (, ).

Although not unhealthy, these side effects can be unpleasant.

Belonging to a class of fibers called FODMAPs, the fibers raffinose and stachyose may worsen symptoms of IBS, a common digestive disorder ().

If you have IBS, avoiding or limiting the consumption of soybeans may be a good idea.

Soy allergy

Food allergy is a common condition caused by a harmful immune reaction to certain components in foods.

Soy allergy is triggered by soy proteins — glycinin and conglycinin — found in most soy products ().

Even though soybeans are one of the most common allergenic foods, soy allergy is relatively uncommon in both children and adults (, ).

SUMMARY In some people, soy products may suppress thyroid function, cause flatulence and diarrhea, and lead to allergic reactions.